Two main techniques are commonly used to fix an implantable orthopedic prosthesis to bone: Techniques using bone cement, and techniques pressing the implant into position. During the former technique, bone cement (typically PMMA, or polymethylmethacrylate) is applied in a dough-like state as a grouting agent between the bone and the implant. The cement flows around the contours of the osteotomy and the implant and into the interstices of cancellous bone. Upon hardening, the cement forms a mechanical interlock between the bone and the outer surface of the implant. In effect, no bone growth into the surface of the implant occurs since cement is used to hold the implant in place.
Although bone cement gives good initial fixation, it has some disadvantages. As one example, the bond between the bone and implant can weaken over time and cause complications. Further, implants affixed with cement can subside after the cement hardens and also cause unwanted problems.
During the technique of pressing the implant into bone, no bone cement is used. Rather the implant is press-fit into a prepared bone cavity that closely approximates the shape of the implant. Long term stability of press-fit implants requires bone to form an interlock with the outer surface of the implant.
Currently, all porous coatings or textured surfaces of orthopedic implants are fabricated using mainly metallic materials and are on metallic substrates. Therefore, the contemporary porous coatings or textured surfaces of orthopedic implants have a much higher elastic modulus than surrounding bone. As a result of this disparity in elastic modulus, bone tissues are likely fractured or crashed because of the micro-motion and press-fit at the bone-implant interface.
Conventional press-fit prosthesis often provide inconsistent long-term fixation to bone because of inadequate interfacial osteogenesis (i.e., bone growth directly to the outer surface of the implant). As such, much effort and research have been devoted to understanding how bone grows into these surfaces; and how these surfaces can be modified to encourage such growth. In particular, much research has been directed toward bone growth into textured or porous surfaces adapted to engage bone.
Bone growth into porous orthopedic implants is generally a two stage phenomenon. In the first stage and immediately after implantation, the pores or outer textured surface of the porous component fills with a blood clot that begins to organize. Here, fibroblasts appear in the clot region and fibrogenesis occurs. Loose connective tissue and capillaries soon replace the clot. At this point, pre-osteoblasts begin to appear in the peripheral pores of the implant. These cells can become osteoblasts or chondroblasts depending upon the environment. If the original pore size of the implant is too small or if the porous structure has been distorted by applied loads, one or more of the above sequence of events can be interrupted or hampered. For example, a smaller pore size (<90 μm) generally leads to the ultimate formation of fibrous tissue, not bone in the implant.
In the second stage, after bone has filled the pores of the implant, the bone begins to remodel. Here, spicules in the implant that experience uniform stress tend to thicken while those spicules that experience no stress or excessive stress (stress concentration) tend to become resorbed.
During this stage, the material properties of the implant are very important. In particular, implants formed from metal and ceramic can have a distinct disadvantage. For example, the modulus of metals and ceramics is so high that the implants do not sufficiently deform under the applied loads. The implant thus does not adequately spread load to surrounding bone (known as “stress-shielding”), and resorption can occur. Further, bone spicules in these porous implants do not experience sufficient loads to thicken. Bone trabeculae in the higher modulus porous materials tend to thin, a situation that also can lead to resorption.
From this discussion, an important conclusion can be drawn: The biomechanical environment established by the implant material and the geometry of the porous substrate can have a profound effect on osteogenesis or osteointegration at the bone-implant interface. Not surprisingly, much effort has been directed toward engineering polymeric coatings for use with implants. For example, some prior implantable prosthetic devices have been coated with porous bio-engineered thermoplastic materials. U.S. Pat. No. 4,164,794, entitled “Prosthetic Devices Having Coatings of Selected Porous Bioengineering Thermoplastics,” teaches a prosthesis having a thermoplastic coating to encourage bone growth into the surface.
Efforts also have been directed toward forming implants entirely of polymeric materials. Polymeric implants can have excellent characteristics for orthopedic, dental, and maxillofacial applications. The transmission of stress to bone in some of these implants more closely mimics the physiological, natural biomechanical environment. U.S. Pat. No. 4,199,864, entitled “Endosseous Plastic Implant Method,” for example, teaches a cast polymeric implant having a porous surface to encourage bone growth. The porous surface was fabricated through polymerization of a powdered polymer-liquid monomer mixture. The prior art, then, has directed much effort to coating metallic implants with a polymer or forming implants entirely from polymers. At the same time, much research has been devoted to bone growth into metals and polymers alike.
It therefore would be advantageous to provide an implantable orthopedic prosthesis that was formed at least partially from a polymer having a porous or textured outer surface to promote osteogenesis.